Thursday, January 9, 2020

Magnitude of Great Lisbon Earthquake may have been lower than previous estimates

The magnitude of the Great Lisbon Earthquake event, a historic and devastating earthquake and tsunami that struck Portugal on All Saints' Day in 1755, may not be as high as previously estimated.
In his study published in the Bulletin of the Seismological Society of America, Joao F. B. D. Fonseca at the Universidade de Lisboa used macroseismic data -- contemporaneous reports of shaking and damage -- from Portugal, Spain and Morocco to calculate the earthquake's magnitude at 7.7. Previous estimates placed the earthquake at magnitude 8.5 to 9.0.
Fonseca's analysis also locates the epicenter of the 1755 earthquake offshore of the southwestern Iberian Peninsula, and suggests the rupture was a complicated one that may have involved faulting onshore as well. This re-evaluation could have implications for the seismic hazard map of the region, he said.
The current maps are based on the assumption that most of the region's crustal deformation is contained in large offshore earthquakes, without a significant onshore component. "While the current official map assigns the highest level of hazard to the south of Portugal, gradually diminishing toward the north, the interpretation now put forward concentrates the hazard in the Greater Lisbon area," said Fonseca.
The 1755 Lisbon earthquake and tsunami event, along with the fires it caused that burned for hours in the city, is considered one of the deadliest earthquake events in history, leading to the deaths of about 12,000 people. The devastation had a significant impact on Portugal's economy and its political power within Europe, and its philosophical and theological implications were widely discussed by Enlightenment scholars from Voltaire to Immanuel Kant.

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